Membranes variable

Factors affecting RO membrane separations and water flux include feed variables such as solute concentration, temperature, pH, and pretreatment requirements membrane variables such as polymer type, module geometry, and module arrangement and process variables such as feed flow rate, operating time and pressure, and water recovery. 

Efforts to fabricate the NS-300 thin-fllm-composlte membrane by continuous machine casting at FllmTec Corporation have been only partially successful. A severe problem of membrane variability was experienced, which was due in part ostensibly to minor variations in machine-made polysulfone support films. This was studied, and it was postulated that, since there was no intermediate porosity zone as the crossllnked polyethylenimine layer in the NS-lOO membrane, the poly(plperazineamide) membranes would be more sensitive to the defects in the underlying polysulfone support (22). 

Table Observed trends that illustrate the effect of changing membrane variables on measurable performance ...

In practice, temperature changes are dealt with by adjusting the operating pressure lower pressure in the warmer summer months and higher pressure in the colder winter months in surface waters. If there are significant variations in temperature between summer and winter, a 2000 ppm TDS at pH 7.6 feed water, 75% recovery, 3 2 1 array with 3 membranes per pressure vessel, FilmTec BW30-400/34 membranes, variable frequency drive... 

In conclusion, PEM is a core part of PEMFCs. The membrane resistance and water content of the membrane is crucial to the fuel cell performance. This model describes the dynamic process of the membrane variables (e.g., the flux density of HjO, the flux density of H+ and water content). 

Fig. 21. Membrane plate press, ie, recessed plate with variable chamber press.

Fibers spun by this method may be isotropic or asymmetric, with dense or porous walls, depending on the dope composition. An isotropic porous membrane results from spinning solutions at the point of incipient gelation. The dope mixture comprises a polymer, a solvent, and a nonsolvent, which are spun into an evaporative column. Because of the rapid evaporation of the solvent component, the spinning dope solidifies almost immediately upon emergence from the spinneret in contact with the gas phase. The amount of time between the solution s exit from the spinneret and its entrance into the coagulation bath has been found to be a critical variable. Asymmetric fibers result from an inherently more compatible solvent/nonsolvent composition, ie, a composition containing lower nonsolvent concentrations. The nature of the exterior skin (dense or porous) of the fiber is also controlled by the dope composition. 

The three streams and associated variables of the RO membrane process are shown in Figure 2b the feed the product stream, called the permeate and the concentrated reject stream, called the concentrate or retentate. The water flow through the membrane is reported in terms of water flux, J. ... 

The intrinsic rejection and maximum obtainable water flux of different membranes can be easily evaluated in a stirred batch system. A typical batch unit (42) is shown in Figure 5. A continuous system is needed for full-scale system design and to determine the effects of hydrodynamic variables and fouling in different module configurations. A typical laboratory/pilot-scale continuous unit using computer control and on-line data acquisition is shown in Figure 6. 

Flux response to concentration, cross flow or shear rate, pressure, and temperature should be determined for the allowable plant excursions. Fouling must be quantified and cleaning procedures proven. The final design flux should reflect long-range variables such as feed-composition changes, reduction of membrane performance, long-term compaction, new foulants, and viscosity shifts. 

An effective way of preventing vent loss is to use one of the many types of variable-volume tanks. These are built under API Standard 650. They may have floating roofs of the double-deck or the singledeck type. There are hfter-roof types in which the roof either has a sldrt moving up and down in an annular hquid seal or is connected to the tank shell oy a flexible membrane. A fabric expansion chamber housed in a compartment on top of the tank roof also permits variation in volume. 

Variable-Volume Filter Presses These membrane filter presses are covered in the section on filtration. Two designs are used for the presses (1) the typical variable-volume filter press has a normal vertical leaf design (2) other presses, such as those provided by Filtra-Systems and Larox, are designed with horizontally arranged leaves. 

Membrane Characterization Membranes are always rated for flux and rejection. NaCl is always used as one measure of rejection, and for a veiy good RO membrane, it will be 99.7 percent or more. Nanofiltration membranes are also tested on a larger solute, commonly MgS04. Test results are veiy much a function of how the test is run, and membrane suppliers are usually specific on the test conditions. Salt concentration will be specified as some average of feed and exit concentration, but both are bulk values. Salt concentration at the membrane governs performance. Flux, pressure, membrane geome-tiy, and cross-flow velocity all influence polarization and the other variables shown in Fig. 22-63. 

Equation (22-106) gives a permeate concentration as a function of the feed concentration at a stage cut, 0 = 0, To calculate permeate composition as a function of 0, the equation may be used iteratively if the permeate is unmixed, such as would apply in a test cell. The calculation for real devices must take into account the fact that the driving force is variable due to changes on both sides of the membrane, as partial pressure is a point function, nowhere constant. Using the same caveat, permeation rates may be calciilated component by component using Eq. (22-98) and permeance values. For any real device, both concentration and permeation require iterative calculations dependent on module geometiy. 

Driving Force Gas moves across a membrane in response to a difference in chemical potential. Partial pressure is sufficiently proportional to be used as the variable for design calculations for most gases of interest, but fugacity must be used for CO9 and usually for Hg... 

As yet, models for fluid membranes have mostly been used to investigate the conformations and shapes of single, isolated membranes, or vesicles [237,239-244], In vesicles, a pressure increment p between the vesicle s interior and exterior is often introduced as an additional relevant variable. An impressive variety of different shapes has been found, including branched polymer-like conformations, inflated vesicles, dumbbell-shaped vesicles, and even stomatocytes. Fig. 15 shows some typical configuration snapshots, and Fig. 16 the phase diagram for vesicles of size N = 247, as calculated by Gompper and Kroll [243]. 

The polymers obtained by this copolymerization [103] show weight average molecular weights upto 2 x 10. Such functionalized copolyethers are of interest for preparation of membranes with variable hydrophilicity and permeability [104]. 

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